U.S. patent number 6,623,720 [Application Number 09/822,609] was granted by the patent office on 2003-09-23 for transition metal carbides, nitrides and borides, and their oxygen containing analogs useful as water gas shift catalysts.
This patent grant is currently assigned to The Regents of the University of Michigan. Invention is credited to Dong Ju Moon, Jeremy Patt, Cory Phillips, Levi T. Thompson.
United States Patent |
6,623,720 |
Thompson , et al. |
September 23, 2003 |
Transition metal carbides, nitrides and borides, and their oxygen
containing analogs useful as water gas shift catalysts
Abstract
Mono- and bimetallic transition metal carbides, nitrides and
borides, and their oxygen containing analogs (e.g. oxycarbides) for
use as water gas shift catalysts are described. In a preferred
embodiment, the catalysts have the general formula of M1.sub.A
M2.sub.B Z.sub.C O.sub.D, wherein M1 is selected from the group
consisting of Mo, W, and combinations thereof; M2 is selected from
the group consisting of Fe, Ni, Cu, Co, and combinations thereof; Z
is selected from the group consisting of carbon, nitrogen, boron,
and combinations thereof; A is an integer; B is 0 or an integer
greater than 0; C is an integer; O is oxygen; and D is 0 or an
integer greater than 0. The catalysts exhibit good reactivity,
stability, and sulfur tolerance, as compared to conventional water
shift gas catalysts. These catalysts hold promise for use in
conjunction with proton exchange membrane fuel cell powered
systems.
Inventors: |
Thompson; Levi T. (Northville,
MI), Patt; Jeremy (Novi, MI), Moon; Dong Ju (Ann
Arbor, MI), Phillips; Cory (Detroit, MI) |
Assignee: |
The Regents of the University of
Michigan (Ann Arbor, MI)
|
Family
ID: |
22714103 |
Appl.
No.: |
09/822,609 |
Filed: |
March 30, 2001 |
Current U.S.
Class: |
423/656;
252/373 |
Current CPC
Class: |
B01J
27/22 (20130101); C01G 49/009 (20130101); C01B
32/90 (20170801); C01G 39/006 (20130101); H01M
8/0668 (20130101); C01G 53/006 (20130101); C01B
32/907 (20170801); B01J 27/24 (20130101); B01J
21/02 (20130101); C01B 3/16 (20130101); C01B
35/04 (20130101); C01G 41/006 (20130101); C01G
51/006 (20130101); H01M 8/0612 (20130101); H01M
2008/1095 (20130101); C01B 2203/1047 (20130101); C01B
2203/1041 (20130101); C01P 2002/54 (20130101); C01B
2203/1076 (20130101); C01P 2004/80 (20130101); C01P
2006/16 (20130101); C01B 2203/1052 (20130101); C01P
2006/12 (20130101); C01B 2203/0283 (20130101); Y02E
60/50 (20130101); C01B 2203/066 (20130101); C01P
2002/72 (20130101); Y02P 20/52 (20151101) |
Current International
Class: |
C01G
39/00 (20060101); C01B 3/00 (20060101); C01B
3/16 (20060101); C01G 49/00 (20060101); C01G
53/00 (20060101); B01J 21/02 (20060101); B01J
21/00 (20060101); B01J 27/22 (20060101); B01J
27/20 (20060101); B01J 27/24 (20060101); C01G
51/00 (20060101); C01G 41/00 (20060101); C01B
003/16 () |
Field of
Search: |
;252/373 ;423/655,656
;502/200,204,207,180,184,185,182,305,313,314,315,316,321,345,338 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Silverman; Stanley S.
Assistant Examiner: Medina; Maribel
Attorney, Agent or Firm: Harness, Dickey & Pierce,
P.L.C.
Government Interests
STATEMENT OF GOVERNMENTAL SUPPORT
This invention was made with Government support under Grant No.
DE-FC02-98EE50538 awarded by the Department of Energy. The
Government has certain rights in this invention.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of U.S. Provisional Application
No. 60/193,553 filed on Mar. 31, 2000, the disclosure of which is
incorporated herein by reference.
Claims
What is claimed is:
1. A method for catalyzing a water gas shift reaction in which
carbon monoxide levels in a hydrogen-containing stream are reduced,
comprising: providing a catalyst having the formula:
2. The invention according to claim 1 wherein the transition metal
comprising M1 does not comprise the transition metal comprising
M2.
3. The invention according to claim 1 wherein the molar ratio of
M1:M2 is 1 or greater: 0 or greater.
4. The invention according to claim 1 wherein M1 is selected from
the group consisting of molybdenum, tungsten, and combinations
thereof.
5. The invention according to claim 1 wherein M2 is selected from
the group consisting of iron, nickel, copper, cobalt, and
combinations thereof.
6. The invention according to claim 1 wherein the catalyst is
substantially sulfur tolerant.
7. The invention according to claim 1 wherein the catalyst causes
the reaction rate of the water shift gas reaction to increase over
time.
8. The invention according to claim 1 wherein the reduction of
carbon monoxide levels occur upstream of a fuel cell.
9. The invention according to claim 8 wherein said fuel cell
comprises a proton exchange membrane.
10. A method for catalyzing a water gas shift reaction in which
carbon monoxide levels in a hydrogen-containing stream are reduced,
comprising: providing a catalyst having the formula:
11. The invention according to claim 10 wherein the molar ratio of
M1:M2 is 1 or greater: 0 or greater.
12. The invention according to claim 10 wherein the catalyst is
substantially sulfur tolerant.
13. The invention according to claim 10 wherein the catalyst causes
the reaction rate of the water shift gas reaction to increase over
time.
14. The invention according to claim 10 wherein the reduction of
carbon monoxide levels occur upstream of a fuel cell.
15. The invention according to claim 14 wherein said fuel cell
comprises a proton exchange membrane.
Description
FIELD OF THE INVENTION
The present invention relates generally to fuel cells, and more
particularly to transition metal carbides, nitrides and borides,
and their oxygen containing analogs (e.g. oxycarbides) useful as
water gas shift catalysts for use in producing hydrogen for
chemical processing and petroleum refining, and reducing the carbon
monoxide content of feeds to fuel cells.
BACKGROUND OF THE INVENTION
The water gas shift (WGS) is an important reaction in the
conversion of fossil fuels into hydrogen for use in processing
chemicals and refining petroleum. An important emerging application
is in the production of hydrogen for fuel cells. Fuel cells
electrochemically convert fuel and oxidant directly into
electricity. Because of their inherent high efficiencies and low
emissions, fuel cells have gained significant interest from
automobile manufacturers and their suppliers. Many manufacturers
favor the use of proton exchange membrane (PEM) fuel cells
operating with hydrogen from the processing of fossil fuels. The
key fuel processing steps are (1) steam reforming and/or partial
oxidation and (2) water gas shift.
Hydrocarbon steam reforming and partial oxidation are the principal
reactions used to generate hydrogen. Hydrocarbon steam reforming is
highly endothermic and usually requires temperatures in excess of
700.degree. C. to be effective (eqn. 1). Performance of the
reformer is very sensitive to the composition of the fuel,
consequently steam reforming is not considered to be very fuel
flexible.
Hydrogen can also be extracted from hydrocarbons via partial
oxidation reactions (see for example eqn. 2). Partial oxidation
reactions are exothermic; however, because the reaction is not
catalyzed, temperatures in excess of 1000.degree. C. are required
to achieve the necessary rates. The product composition is
regulated by controlling the amount of O.sub.2.
In autothermal reforming, partial oxidation is coupled with steam
reforming. The relative contribution of steam reforming versus
partial oxidation can be controlled by choice of catalyst and
operation conditions. For a given feed, the reaction temperature is
lower than that for partial oxidation alone. Compared to steam
reforming, autothermal reforming can be carried out in a smaller
reactor volume, starts faster, and responds more quickly to control
actions or changes in feed conditions.
The water gas shift reaction (eqn. 3) is well established for
producing hydrogen and decreasing the CO content to less than
1%.
Carbon monoxide removal is critical because many catalysts are
poisoned by CO. For example, the noble metal electrocatalysts in
PEM fuel cells are susceptible to poisoning by as little as 10-100
ppm CO. The poisoning problem is exacerbated by the operating
constraints imposed by commercial membrane materials. Present PEM
fuel cells must be operated under conditions which avoid drying out
the membrane. This essentially excludes operating the fuel cell at
the higher temperatures where Pt oxidizes CO. The water gas shift
reaction is typically carried out in two stages using Fe--Cr
catalysts in the high temperature stage and Cu--Zn--Al catalyst in
the low temperature stage.
Presently employed catalysts lack sufficient activity and
durability for many portable and automotive applications.
Furthermore, presently available catalysts are very sensitive to
sulfur compounds, a common contaminant in modern transportation
fuels.
Therefore, there exists a pressing need for water gas shift
catalysts that are highly active, durable, and sulfur tolerant.
These materials would be especially well suited for use in
conjunction with PEM fuel cells for automotive applications.
BACKGROUND REFERENCES
U.S. Pat. No. 3,666,682 to Muenger, the entire specification of
which is incorporated herein by reference, discloses a water gas
shift conversion process in which a feed gas mixture is subjected
to successive contacts with catalyst and the temperature of the
reacting gases contacting the shift conversion catalyst is
controlled by indirect concurrent heat exchange with the feed gas
mixture.
U.S. Pat. No. 3,974,096 to Segura et al., the entire specification
of which is incorporated herein by reference, discloses that
hydrogen is produced by reacting carbon monoxide with steam at a
temperature of at least 200.degree. F. in the presence of a
supported catalyst containing: (1) at least one alkali metal
compound derived from an acid having an ionization constant below
1.times.10.sup.-3, (2) a metallic hydrogenation-dehydrogenation
material, and (3) a halogen moiety. The ratio of metal component to
alkali metal compound, each calculated on the basis of the oxide
thereof, ranges from 0.0001 to about 10 parts by weight per part by
weight of the alkali metal compound. The halide constituent is
present in amounts in excess of about 0.01 weight %, based on total
catalyst. A preferred catalyst composition comprises potassium
carbonate, a mixture of cobalt and molybdenum oxides and combined
chlorine contained on an alumina support.
U.S. Pat. No. 4,172,808 to Bohm et al., the entire specification of
which is incorporated herein by reference, discloses a process for
the production of a tungsten carbide catalyst by carburization of
tungsten oxides, comprises, directing a mixture of carbon monoxide
and carbon dioxide over tungsten oxide while heating it in a heated
reactor at a heating rate and gas flow rate such that the reduction
of the tungsten oxide occurs more slowly than the diffusion of the
carbon into the tungsten and into tungsten carbide which is formed
during the reaction with the diffusion being faster than the
separation of carbon from the gaseous phase according to the rate
of adjustment of the Boudouard equilibrium. The carbon monoxide is
charged at a rate of 560 l/h and the carbon dioxide is charged at a
rate of 40 l/h and, after a reactor containing the sample of
tungstic acid is positioned in a closed reactor, the reactor is
flushed with the gases for around ten minutes and then placed into
a muffle furnace. The reactor is heated to a temperature of
670.degree. C. in the furnace and the temperature is then reduced
to a reaction temperature of 620.degree. C. First, all of the water
is eliminated, and then there is a reduction of the tungsten oxides
and a diffusion of the carbon into tungsten or into tungsten
carbide which is formed. The reduction of the tungsten oxides
occurs more slowly than the diffusion of the carbon, but faster
than the deposition of the carbon from the gaseous phase.
U.S. Pat. No. 4,219,445 to Finch, the entire specification of which
is incorporated herein by reference, discloses a process of
preparing methane-containing gas comprising contacting carbon
monoxide and hydrogen in the presence of a catalyst containing
tungsten carbide. Various tungsten carbide-containing alumina gel
catalysts are also disclosed.
U.S. Pat. No. 4,271,041 to Boudart et al., the entire specification
of which is incorporated herein by reference, discloses a high
specific surface area molybdenum oxycarbide catalyst. They are
prepared by the vapor condensation of molybdenum hexacarbonyl and
catalyze the reaction of hydrogen and carbon monoxide to form
hydrocarbons. Carburization of the molybdenum oxycarbides increases
their activity in the carbon monoxide-hydrogen reaction.
U.S. Pat. No. 4,325,842 to Slaugh et al., the entire specification
of which is incorporated herein by reference, discloses a process
for preparing a supported molybdenum carbide composition which
comprises impregnating a porous support with a solution of
hexamolybdenum dodecachloride, drying the impregnated support and
then heating in a carbiding atmosphere at a temperature of about
650.degree.-750.degree. C.
U.S. Pat. No. 4,325,843 to Slaugh et al., the entire specification
of which is incorporated herein by reference, discloses a process
for preparing a supported tungsten carbide composition which
comprises first forming a supported tungsten oxide composition,
converting the oxide to the nitride by heating in an ammonia
atmosphere, and then converting the nitride to the carbide by
heating in a carbiding atmosphere.
U.S. Pat. No. 4,789,534 to Laine, the entire specification of which
is incorporated herein by reference, discloses transition metal
carbides in which the carbon is in excess and is covalently bound
to the metal are produced by pyrolyzing transition metal amides
that have two or more metal atoms, such as hexakis (dimethylamido)
ditungsten or dimolybdenum.
U.S. Pat. No. 4,808,563 to Velenyi, the entire specification of
which is incorporated herein by reference, discloses a catalyst
which comprises a molybdenum-tungsten-containing complex
represented by the formula Mo.sub.a W.sub.b M.sub.c A.sub.d
O.sub.e, wherein M is selected from the group consisting of one or
more metals selected from any of Groups IB, IIB, IVB, VB or VIII of
the Periodic Table and/or one or more of Y, Cr, Mn, Re, B, In, Ge,
Sn, Pb, Th or U, or a mixture of two or more of the metals in said
group; A is at least one metal selected from the group consisting
of alkali metals, alkaline earth metals, Lanthanide series metals,
La, T1, or a mixture or two or more of the metals in said group; a
is a number in the range of from about 1 to about 200; b is a
number in the range of from about 1 to about 200; with the proviso
that either Mo or W is in excess of the other, the ratio of a:b
being about 4:1 or greater, or about 1:4 or less; c is a number
such that the ratio of c:(a+b) is in the range of from 0:100 to
about 10:100; d is a number such that the ratio of d:(a+b) is in
the range of from 0:100 to about 75:100; and e is the number of
oxygens needed to fulfill the valence requirements of the other
elements. A process for converting gaseous reactants comprising
methane and oxygen to higher order hydrocarbons using the foregoing
catalyst is also disclosed.
U.S. Pat. No. 4,812,434 to Pohlmann et al., the entire
specification of which is incorporated herein by reference,
discloses an exhaust gas catalyst, wherein it consists of about 50
to about 95% by weight of silicon carbide and about 5 to about 50%
by weight of an alloy of silicon with one or more metals of the
group copper, iron, cobalt, nickel, chromium, vanadium, molybdenum,
manganese, zinc, silver, platinum, palladium or other
catalytically-active metals, the catalytically-active surface of
which has optionally been activated by oxidation and/or chemical
after-treatment.
U.S. Pat. No. 4,851,206 to Boudart et al., the entire specification
of which is incorporated herein by reference, discloses methods and
compositions produced thereby concerning the preparation and use of
high specific surface area carbides and nitrides. The carbides and
nitrides can be obtained by thermal reduction of oxides in the
presence of a source of carbon and nitrogen respectively, with
relatively slow progressive temperature increases prior to
completion of the reaction, followed by quenching. Novel metastable
carbides can be obtained by carburization of nitrides having high
surface area, which nitrides can be prepared by the above-described
process.
U.S. Pat. No. 5,039,503 to Sauvion et al., the entire specification
of which is incorporated herein by reference, discloses that carbon
monoxide is reacted with water vapor and converted into hydrogen
and carbon dioxide, in the presence of a thio-resistant catalyst
which comprises an active phase deposited onto a support, said
active phase comprising molybdenum, vanadium or tungsten, and a
cobalt and/or nickel promoter therefor, and said support comprising
cerium oxide or zirconium oxide. The reaction mixture includes
carbon monoxide, hydrogen, water and compounds of sulfur, wherefrom
hydrogen is selectively produced in increased amounts.
U.S. Pat. No. 5,321,161 to Vreugdenhil et al., the entire
specification of which is incorporated herein by reference,
discloses that nitrides can be hydrogenated to amines by heating
the nitrile in the presence of hydrogen and a tungsten carbide
catalyst, such as are formed by the calcination of a tungsten salt
with an acyclic compound containing-nitrogen-hydrogen bonding.
U.S. Pat. No. 5,444,173 to Oyama et al., the entire specification
of which is incorporated herein by reference, discloses bimetallic
oxynitrides and nitrides which have catalytic properties comprise
two transition metals selected from Groups IIIB to VIII of the
Periodic Table of the Elements. Preferably, one metal is either
molybdenum or tungsten. The other can be tungsten or molybdenum,
respectively, or another transition metal, such as vanadium,
niobium, chromium, manganese, cobalt, or nickel. They have a face
centered cubic (fcc) arrangement of the metal atoms and have a
surface area of no less than about 40 m.sup.2 /gm.
U.S. Pat. No. 5,468,370 to Ledoux et al., the entire specification
of which is incorporated herein by reference, discloses a catalyst
for chemical and petrochemical reactions and a process for its
production. The catalyst comprises an oxide of one of the
transition metals, rare earth elements, or actinide elements, e.g.,
molybdenum, having on its surface carbides and oxycarbides, the
core being the metal or the metal oxide. In the process for
catalyst production, the reaction gas mixture containing carbon
products is passed onto the oxide, leading to a progressive
carburization of the surface of the oxide and to a progressive
increase in the efficiency of the catalyst.
U.S. Pat. No. 5,821,190 to Kurabayashi et al., the entire
specification of which is incorporated herein by reference,
discloses a catalyst and method for purifying exhaust gases, having
superior performance of NOx purification to exhaust gases
containing oxygen and nitrogen oxides, particularly superior
performance of NOx elimination to exhaust gases from lean-burn
engines with excess oxygen, and a wider effective temperature range
of NOx elimination, and also superior heat resistance at high
temperature. The catalyst for purifying exhaust gases comprises, as
indispensable contents, iridium and alkaline metal loaded on a
carrier which is at least one selected from metal carbide and metal
nitride, or these and at least one element selected from the group
consisting of alkaline earth metal elements and rare earth metal
elements.
SUMMARY OF THE INVENTION
In accordance with one embodiment of the present invention, a
catalyst for catalyzing the water gas shift reaction is provided,
comprising the formula:
M1.sub.A M2.sub.B Z.sub.C O.sub.D wherein M1 is a transition metal;
M2 is a transition metal; A is an integer; B is 0 or an integer
greater than 0; Z is selected from the group consisting of carbon,
nitrogen, boron, and combinations thereof; C is an integer; O is
oxygen; and D is 0 or an integer greater than 0.
In accordance with another embodiment of the present invention, a
catalyst for catalyzing the water gas shift reaction is provided,
comprising the formula:
In accordance with another embodiment of the present invention, a
method is provided for catalyzing the water gas shift reaction in
which carbon monoxide levels in a hydrogen-containing stream are
reduced, comprising: providing a catalyst having the formula:
In accordance with another embodiment of the present invention, a
method is provided for catalyzing the water gas shift reaction in
which carbon monoxide levels in a hydrogen-containing stream are
reduced, comprising: providing a catalyst having the formula:
wherein M1 is selected from the group consisting of molybdenum,
tungsten, and combinations thereof; M2 is selected from the group
consisting of iron, nickel, copper, cobalt, and combinations
thereof; A is an integer; B is 0 or an integer greater than 0; Z is
selected from the group consisting of carbon, nitrogen, boron, and
combinations thereof; C is an integer; O is oxygen; D is 0 or an
integer greater than 0; and exposing the hydrogen-containing stream
to the catalyst for a sufficient period of time to reduce the
carbon monoxide levels in the hydrogen-containing stream.
A more complete appreciation of the various embodiments and aspects
of the present invention and the scope thereof can be obtained from
a study of the accompanying drawings, which are briefly summarized
below, the following detailed description of the invention, and the
appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graphical illustration showing the carbon monoxide
consumption rates of several exemplary transition metal carbide
water gas shift catalysts of the present invention as compared to
several commercial water gas shift catalysts, in accordance with
one aspect of the present invention; and
FIG. 2 is a graphical illustration showing the X-ray diffraction
patterns for as-prepared Mo.sub.2 C of the present invention and
the same material after catalyzing the water gas shift at
temperatures up to 400.degree. C., in accordance with one aspect of
the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In accordance with one embodiment of the present invention, mono-
and/or bimetallic transition metal carbides, nitrides and borides,
and their oxygen containing analogs (e.g. oxycarbides) are
provided. These compounds are particularly useful as water gas
shift catalysts for PEM fuel cells. Transition metals are generally
defined as those elements found in Groups IIIB(3) through IIB(12)
of the Periodic Table of the Elements.
In accordance with a preferred embodiment of the present invention,
the mono- and/or bimetallic transition metal compounds have the
general formula M1.sub.A M2.sub.B Z.sub.C O.sub.D, wherein M1 is
selected from the group consisting of Mo, W, and combinations
thereof; M2 is selected from the group consisting of Fe, Ni, Cu,
Co, and combinations thereof; Z is selected from the group
consisting of carbon, nitrogen, boron, and combinations thereof; A
is an integer; B is 0 or an integer greater than 0; C is an
integer; 0 is oxygen; and D is 0 or an integer greater than 0. It
should be appreciated that the molar ratios of M1:M2 can vary quite
considerably, as will be described herein. However, in a preferred
embodiment, the molar ratio of M1:M2 is 1 or greater: 0 or
greater.
The various transition metal carbides (TMC's), nitrides (TMN's) and
borides (TMB'S), and their oxygen containing analogs of the present
invention were synthesized from solid oxide precursors. For
example, the TMC's were prepared as follows. The oxides were
prepared from dried mixtures of ammonium and/or nitrate salts.
These salts were dissolved in warm deionized water. The liquid was
slowly evaporated and the remaining solid was calcined in dry air
for 3 hrs at 500.degree. C. and sieved to retain material with a
mesh size of -60+230. The calcination temperature was determined
via thermal gravimetric analysis (TGA).
The oxide was then carburized in a temperature programmed manner
using an equimolar CH.sub.4 /H.sub.2 mixture flowing at 50-300
cm.sup.3 /min. Typically, 1-4 g of the oxide was loaded on a quartz
wool plug in a quartz straight tube reactor. Because graphitic
carbon can block catalytically active sites, special care was taken
to avoid deposition of excess carbon. Solid-state reaction pathways
and appropriate final temperatures (T.sub.f) were determined using
TGA in conjunction with X-ray diffraction (XRD). Presence of the
Group VIII metal caused a reduction in the temperature required to
reduce then carburize the oxide. For example, the oxide containing
Mo and Ni carburized at a temperature more than 50.degree. C. lower
than that required to carburize the Mo oxide. The addition of Cu
also resulted in a substantial reduction in the temperature
required to accomplish the first transformation. It should also be
noted that the rate of carbon deposition is accelerated for
materials containing the Group VIII metal.
Temperature programs used to synthesize the TMC's of the present
invention consisted of linearly heating the oxide at a rate of
600.degree. C./hr to 300.degree. C., then at 60.degree. C./hr to
the final temperature (T.sub.f). Following a 2 hr soak period, the
product was quenched to room temperature and passivated for 4 hrs
in a mixture of 1% O.sub.2 /He flowing at 30 cm.sup.3 /min. This
passivation step was necessary to prevent pyrophoric oxidation of
the carburized product upon contact with air. The final
temperatures and bulk phases are summarized in Table I, below:
TABLE I Avg. M1:M2 Main Surface Pore Catalyst Molar Oxide T.sub.f
Phases Area Size Formula Ratio Phases (.degree. C.) Present
(m.sup.2 /g) (nm) Mo.sub.2 C 1:0 MoO.sub.3 615 Mo.sub.2 C 61 3
Mo.sub.7 Fe.sub.4 C 1.75:1 Fe.sub.2 (MoO.sub.4).sub.3 590 Mo.sub.2
C, 23 19 Fe, MoO.sub.2 Mo.sub.4 Fe.sub.7 C 1:1.72 Fe.sub.2
(MoO.sub.4).sub.3 548 Mo.sub.2 C, 32 24 Fe, MoO.sub.2 Mo.sub.11
Ni.sub.6 C 1.84:1 NiMoO.sub.4, 571 Mo--Ni 87 8 MoO.sub.3 carbide,
Ni, MoO.sub.2 Mo.sub.14 Ni.sub.23 C 1:1.64 NiMoO.sub.4, 550 Mo--Ni
33 11 MoO3 carbide, Ni, MoO.sub.2 Mo.sub.2 CuC 1.99:1 CuMoO.sub.4,
620 Cu, 44 8 MoO.sub.3 Un- known phase* Mo.sub.2 Cu.sub.3 C 1:1.5
CuMoO.sub.4, 620 Cu, 28 5 MoO.sub.3 Un- known phase* *Pattern did
not match any patterns in the Joint Committee on Powder Diffraction
Standards (JCPDS) Tables.
The predominant phase in most of the materials was a carbide;
however, some materials contained small amounts of metal and oxide.
The diffraction pattern for the Mo formulation indicated phase pure
Mo.sub.2 C.
The Brunner Emmet Teller (BET) surface areas and pore size
distributions were determined by N.sub.2 physisorption. Density
Functional Theory was used to estimate the pore size distributions
and average pore sizes. A summary of the results is provided in
Table I.
Prior to measurement of the reaction rates, the catalysts were
pretreated. Reduction temperatures for pretreatment of the
catalysts were determined based on the results of temperature
programmed reduction (TPR). A 100 mg sample of the catalyst was
heated to 600.degree. C. at 600.degree. C./hr in 5% H.sub.2 /Ar
while the effluent composition was monitored. Reduction at
400.degree. C. appeared to be adequate for most of the carbides
although optimization of the pretreatment conditions could result
in substantial improvements in performance.
The water gas shift reaction rates and product selectivities were
measured using synthetic CH.sub.4 steam reformer exhaust mixtures.
A 0.15 g sample of each catalyst was diluted with approximately 0.1
g of silica gel to attain a constant bed volume of 0.34 cm.sup.3.
The carbide catalysts were reduced at 400.degree. C. for 4.5 hrs in
H.sub.2. Rates for commercial low temperature shift (LTS) (60
m.sup.2 /gr; Cu--Zn--Al from United Catalysts Inc.) and high
temperature shift (HTS) (55 m.sup.2 /gr; Fe--Cr from United
Catalysts Inc.) catalysts were also measured for comparison
purposes. The LTS catalyst was reduced at 200.degree. C. in 2%
H.sub.2 /N.sub.2 for 4.5 hrs per recommendations from the
manufacturer. The HTS catalyst was reduced in a stream containing
55% H.sub.2, 5% CO, 10% CO.sub.2, 28% H.sub.2 O and 3% CH.sub.4 for
4.5 hrs at 370.degree. C. Reaction rates were measured for
temperatures between 200-400.degree. C. The reactant gases were
delivered by mass flow controllers and the deionized water was fed
using a Rainin Rabbit HPLC pump. An SRI Model 8610C gas
chromotograph equipped with Porapak Q and molecular sieve columns
and a thermal conductivity detector was used to measure the
reactant and effluent compositions. The total reactant flow rate
was approximately 155 cm.sup.3 /min yielding gas hourly space
velocities of approximately 25,000 hr.sup.-1. The conversion was
limited to 20% to simplify data analysis and avoid the equilibrium
limit. The reaction rates reached steady-state after approximately
1 hr on stream and were reproducible to within 10% during
subsequent runs. The blank run for a 0.34 cm.sup.3 bed of silica
gel showed no WGS activity. There was also no evidence of
methanation activity for any of the catalysts under the conditions
employed.
Reaction rates measured using a feed stream containing 62.5%
H.sub.2 (99.99% pure), 5.7% CO (99.5% pure) and 31.8% H.sub.2 O are
shown in FIG. 1. Activities for the TMC catalysts were comparable
or superior to those of commercial Fe--Cr (UCI-HTS) and Cu--Zn
(UCI-LTS) catalysts. The most active carbide significantly
outperformed the Cu--Zn catalyst.
The turnover frequency for the Mo.sub.2 C catalyst was estimated to
be 0.05 sec.sup.-1 at 270.degree. C. based on site densities
measured via O.sub.2 chemisorption at -77.degree. C. For
comparison, the turnover frequency for the LTS catalyst was 0.04
sec.sup.-1 at 270.degree. C. (based on O.sub.2 uptake). The oxygen
uptake for the LTS catalyst yielded a Cu surface area of 11.5
m.sup.2 /g when the O.sub.2 cross-sectional area was assumed to be
approximately 0.2 nm.sup.2 /molecule. This surface area is nearly
identical to that supplied by the manufacturer suggesting that this
catalyst was properly pretreated.
The performance of the carbides was also compared to that of a
Pd/CeO.sub.x catalyst (10 wt. % Pd and a surface area of 120
m.sup.2 /gr). This type of catalyst is reported to catalyze the WGS
reaction with high rates (Bunluesin et al., 1998). The WGS rates
were determined using a reactant gas containing 55% H.sub.2, 5% CO,
10% CO.sub.2, 28% H.sub.2 O and 3% CH.sub.4. The Pd/CeO.sub.x
catalyst was calcined at 600.degree. C. for 12 hrs prior to the
rate measurements. The results for measurements at 270.degree. C.
are summarized in Table II, below:
TABLE II Initial Rate End-Run Rate Activation Catalyst
(.mu.mol/g.multidot.sec) (.mu.mol/g.multidot.sec) Energy (kcal/mol)
Mo.sub.2 C 5.5 7.1 17 UCI-LTS 3.6 3.1 16 Pd/CeO.sub.x 8.5 2.8
18
The initial CO consumption rate for the Pd/CeO.sub.x catalyst was
very high; however, this material deactivated to a level lower than
that of the carbide and LTS catalysts. Apparent activation energies
observed for the Cu/Zn/Al LTS and Pd/CeO.sub.x catalysts are
consistent with values reported in the literature (Mellor et al.,
1997; Bunluesin et al., 1998).
Rates for the carbides were also measured after thermal cycling. In
these experiments, the catalyst was maintained in the reactant
mixture at room temperature overnight between cycles. Within
experimental error, there was no deactivation for the TMC catalysts
suggesting that these materials are very durable. X-ray diffraction
patterns before and after use in the WGS reactor are illustrated in
FIG. 2 for the Mo.sub.2 C catalyst.
Thus, the present invention has yielded a new class of WGS
catalysts. These carbide-, nitride-, and boride-based catalysts are
expected to reduce the PEM fuel cell fuel processor size and cost
as a consequence of their exceptional activities and durabilities,
and tolerance to sulfur. Improvements in catalyst activity may be
secondary to the demonstration of poison tolerance. The sulfur
content in streams produced via reformation of transportation fuels
is expected to be 0.5-1 ppm. At these levels most metal catalysts
rapidly deactivate. Catalyst beds employing Cu/Zn catalysts are
typically designed to be 2.5-3 times larger than necessary due to
sulfur poisoning. Consequently, a sulfur tolerant catalyst that is
as active in the presence of sulfur as presently available Cu/Zn
catalysts are in the absence of sulfur would yield at least a 50%
reduction in bed size. By analogy, with high surface area Mo that
is produced by reduction of MoO.sub.3 with H.sub.2, it is
envisioned that costs for the bulk carbide powders will be low.
Anticipated performance characteristics for the carbide catalysts
of the present invention are compared to those for other candidate
WGS catalysts in Table III, below:
TABLE III Performance Carbide Cu/Zn/Al Pd/CeO.sub.x Characteristic
Catalysts Catalysts Catalysts Activity Good Good Excellent
Durability Excellent Poor Good Sulfur Tolerance Good Poor Poor Cost
Good Good Poor
It is believed that the carbides, nitrides and borides, and their
oxygen containing analogs of the present invention offer the best
combination of properties for use in fuel cell powered
vehicles.
The foregoing detailed description shows that the preferred
embodiments of the present invention are well suited to fulfill the
objects above-stated. It is recognized that those skilled in the
art may make various modifications or additions to the preferred
embodiments chosen to illustrate the present invention without
departing from the spirit and proper scope of the invention.
Accordingly, it is to be understood that the protection sought and
to be afforded hereby should be deemed to extend to the subject
matter defined by the appended claims, including all fair
equivalents thereof.
* * * * *